feat: dedup downloads by (url, format_id) index, reuse existing files

frontend/src/views/Home.vue

@@ -94,8 +94,17 @@ async function startDownload() {
       url: url.value, format_id: selectedFormat.value, quality: selectedFormat.value
     })
     taskId.value = res.data.task_id
-    statusText.value = 'Starting download...'
-    pollStatus()
+    if (res.data.status === 'done') {
+      // Already downloaded — skip polling, show save button immediately
+      progress.value = 100
+      statusText.value = '✅ Already downloaded'
+      downloadReady.value = true
+      downloadUrl.value = `/api/file/task/${res.data.task_id}`
+      downloading.value = false
+    } else {
+      statusText.value = 'Starting download...'
+      pollStatus()
+    }
   } catch (e) {
     error.value = e.response?.data?.detail || 'Failed to start download'
     downloading.value = false